1. Field of the Invention
The present invention relates to an image processing method of and an image processing apparatus for executing image processing on digital image data obtained by a digital electronic still camera (which is hereinafter simply referred to as a digital camera).
2. Description of the Related Art
In a digital camera, an image obtained by photographing is recorded as digital image data (having values corresponding to luminance of a subject, in which normally white is 255, and black is 0) on an internal memory incorporated into the digital camera and on a recording medium such as an IC card. Based on the recorded digital image data, the photographed image can be outputted to a printer and a monitor. When printing the image taken by the digital camera, it is expected that the image has quality as high as a photo printed from a negative film.
Therefore, the digital camera incorporates an auto white balance (AWB) function, an auto exposure (AE) control function, and an image processing function. A method of executing the control so that two chrominance (color difference) signals obtained from a mean value of color signals of the imaged areas becomes “0”, is disclosed as the AWB function for a video camera in, e.g., Japanese Patent Application Laid-Open Nos. Sho 60-20993 and Hei 3-198484. Further, a method of controlling the exposure by comparing a mean value of the luminance signals of all the imaged areas with a mean value of luminance signals of the selected areas, is disclosed as the AE function in, e.g., Japanese Patent Application Laid-open No. Hei 7-75006.
On the other hand, there has hitherto been known a method of reading a color original image and converting into digital image data, and modifying the digital image data to have a proper gradation, density and color on the basis of image characteristic values such as a maximum reference density, a minimum reference density and a histogram obtained from the digital image data (refer to e.g., Japanese Patent Application Laid-open No. Sho 56-87044). Further, there is also proposed a method of creating a conversion table (look-up table: LUT) for reproducing the original image with fidelity from density signals of the digital image data gained by pre-scanning the color original image, then converting the digital image data obtained by main scan using the conversion table LUT, and thus modifying the digital image data (refer to, e.g., Japanese Patent Application Laid-open No. Hei 6-152962). Disclosed further is a method of creating a digital print by converting a negative film image into digital image data (refer to, e.g., Japanese Patent Application Laid-open No. Sho 60-14570). This method involves creating rough image data for showing a light intensity by pre-scanning the negative film, obtaining density data by logarithmically converting the rough image data, converting the density data into such image data as to obtain a proper color and density through a first conversion table LUT, further converting the thus converted image data into positive image data through a second conversion table LUT, and thus modifying the image data taking into consideration characteristics of a copying machine and a copy material (photosensitive material) for copying the image data.
All the subject is recorded as an image on the negative film by photographing excluding a case where extreme over- and under-exposures happen. Therefore, the digital print from the negative film image can utilize the negative film image fully, arbitrarily or selectively for image reproduction.
As described above, the digital camera incorporates the AWB function, the AE function and the image processing function, and the digital image data acquired by the camera is already subjected to the image processing as explained above. Hence, the digital image data is inputted directly to the copying machine such as a printer, whereby the image can be reproduced. The AWB and AE functions are, however, different in their performances depending on a type of the digital camera, or might not be added thereto in some cases. The copying machine for outputting the images of a variety of digital cameras is incapable of obtaining the print images each exhibiting a uniform quality at all times.
A possible solution to this is, as disclosed in, e.g., aforementioned Japanese Patent Laid-open No. Hei 6-152962, to modify the digital image data. It does not assured, however, that the digital image data acquired by the digital camera is always photographed under proper conditions, and besides the digital image data has a large quantity of noises with respect to a shadow image area as well as being rough in terms of image information. Hence, a high-quality image can not be reproduced even by applying, as the aforementioned conventional method, the method as it is of modifying the digital image data obtained by reading the original image photographed as original copy or reading the negative film.
Therefore, there is proposed a method of creating a histogram of the digital image data acquired by the digital camera, obtaining the minimum value, the maximum value (i.e., the shadow point and the highlight point in the image) and the median value of the histogram, obtaining a gray balance from the minimum and maximum values, then obtaining a gamma curve from the minimum and median values, and converting the digital image data (refer to, e.g., Japanese Patent Application Laid-open No. Hei 9-121361). Proposed also is a method of obtaining effective pixels from brightness and a tint per pixel of the digital image data obtained by the digital camera, setting the highlight point and the shadow point in the image from a cumulative histogram of mean density value of the effective pixels, and outputting them to a recording medium such as a CRT and a CD-ROM (refer to, e.g., Japanese Patent Application Laid-open No. Hei 9-238257).
Those methods are, however, contrived to modify the gradations and the colors of the whole digital image data from the highlight point and the shadow point in the image, and the digital image data is based on a premise that a principal image area has a proper value with an adequate exposure. In some of the actual digital cameras, it might often happen that the image data is created with an inadequate exposure such as an overexposure, an underexposure and a mis-modulation of light of an electronic flash etc, and it is of much importance to modify the density of the output image.